Organic peroxides are the industry standard for polymerizing unsaturated polyester resins. Depending on the peroxide structure, cure can be achieved at room temperature to elevated temperatures of 180° C. The desire to cure at low temperature (below 120° C.) is almost uniformly desired. In addition to the obvious energy savings, throughput can be significantly increased by not having to raise the temperature of the article to be cured. Alternate low temperature initiating systems would include diazo compounds as well. The drawback to low temperature curing peroxides or diazo compounds is the stability of the peroxide/diazo compound. Many of the peroxides must be stored at low temperatures and shipped under refrigerated conditions. The thermal instability and handling is a well known danger to employees who use these materials.
Promoters can also be used in combination with peroxides to lower cure temperature. It is common practice to use promoters, such as cobalt, iron, and manganese derivatives to accelerate cure at low temperature. Amine, acetoacetates and amides are also used in combination with metals such as cobalt to promote peroxide decomposition and give a high radical flux needed for cure.
Benzopinacol has been known for some time as a suitable radical polymerization initiator. However, the reactivity and end product properties have not been sufficient enough to overcome the extra preparation expense compared to peroxide based radical initiators. To improve reactivity and solubility, the potassium/sodium salts of benzopinacol have been reacted with di-, tri-, and tetra chlorosilanes or polyorganosilane/siloxane materials. These products have had limited commercial success however they have never been shown to significantly lower cure temperature in unsaturated polymers.
A polyurethane derivatized benzopinacol initiator is known and was reported to behave as a “living” catalyst. The reactivity is also similar to benzopinacol itself. Similar work was shown by Chen, et. al. (European Polymer Journal, 36 (2000) 1547-1554) using monofunctional isocyanates such as phenylisocyanate. These initiators were also found to be “living” catalysts.
Bromoacetyl derivatives of benzopinacol are known as flame retardant initiators for the polymerization of unsaturated polyester (UPE) systems. Additionally, phosphorus and silyl ethers of benzopinacol as flame retardant initiators for UPE systems have been reported.